How can I tell whether old caulking is silicone vs latex acrylic?

I have serious leakage under a wall of windows. The windows are at least 5 feet above ground. I have removed the inside drywall on the underside of the windows as well as the vapor barrier and insulation to find the problem areas. The outside is aluminum siding, on the inside I am now looking at OSB sheathing. From the inside, I can see that water is coming through at the window ledge level, I can’t make out exactly where. I have filled all the holes I can see with Mono Ultra caulk but I still have leakage.

I have now bought a product called “Exterior only – Mulco – Supra Expert – Thermoplastic sealant”. Do I have to remove the old caulk?

I don’t know for sure what all caulk has been used on this, it was built some 15 years ago and has been caulked several times by different people. How can I tell if silicone was used?

Distinguishing between pure silicone caulk and regular paintable caulking isn't easy to describe, I will do my best.

1) Silicone caulking is very rubbery and a little spongy. Regular caulking is harder (less spongy) and will harden more over time.

2) Water beads off silicone. Try wiping the caulk with a rag and water. Make sure to wipe off any dust and observe if the water separates from the caulking.

3) Silicone isn't paintable. If the caulking is painted, try scraping it with your fingernail. The paint will be easily removed.

Beyond the feel of the caulking there isn't any test that I no of. Re-caulking the windows over all the existing caulking's could be a little risky. You can't tell for sure what has been used over the years. I recommend removing as much of the old caulk as possible then re-caulk.

For the leak around the window you should remove all the drywall from around the window use a hose on the window and find where the water incoming in from. It maybe higher than the window it maybe from the roof or anything from roof to window.

Anonymous above may well be right. I had a situation like yours. The water came from a leak in the roof about 10' away. It made its way along the roof deck and rafters down to the outside wall.

Check for water stains on all wood surrounding the windows including the headers. Any sign of water stain there should make you check higher,like in the soffit or attic. OSB is bad for exterior surfaces like outside wall cladding or roof-decks. Once wet from frequent leakage, it will expand and never go back and eventually lose all strength. Look for signs of swelling of the OSB too.

As to the caulking, that has been expertly answered by Karl Crowder, so I won't address that. So, first of all find the source of the leak.